2013-2014

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2013 - 2014 Physics Colloquia

Unless otherwise noted, all physics seminars and colloquia are held on Thursdays from 4:45 to 6:00, in lecture room 3 of Merrill Science Center. Tea and snacks will be served before seminars at 4:15 in 204 Merrill. If you would like to be mailed seminar announcements, please send an email to Ellen Feld.

The physics student of the nineteenth century learned physics though wonderful lecture demonstrations. Here are examples drawn from the fields of electrostatics and oscillations and waves, all illustrated with images drawn from my collection. There is some physics, some history and some art in the study of sparks and wiggles.

Sept. 26 - Dr. Danielle Braje, MIT Lincoln Laboratory

"Quantum Sensing: Harnessing the Power of Quantum Mechanical Systems to Make the World's Most Precise Measurements"

We live in a digital world. Digital clocks, digital circuits, digital calipers, digital communication, digital fingerprinting, digital signal processing. We interact with the world under the assumption that everything digital is better. Here I present an alternative to digital: Quantum mechanical systems (qubits) use a complex phase relationship between the “digital” states to perform accurate measurements. By using quantum mechanical systems such as atoms or quantum defects, exquisitely precise measurements can be made with unique control. I will present results from at atom interferometer magnetometer and show similar measurements we are planning in nitrogen vacancies (a quantum defect or color center in the diamond lattice).

"Solid State Geophysics under Extreme Environments: From Electronic Spin Transitions to Earth’s Interior"

Earth’s interior is subject to crushing pressures of up to 360 GPa and scorching temperatures of up to 6000 K that can only be directly accessed by limited seismic and geochemical probes. Laboratory studies on properties of candidate planetary materials under extreme environments is thus crucial to understanding a plethora of outstanding phenomena in the deep Earth ranging from volcanic eruptions to mantle convections, from geomagnetic fields to core geodynamo, from global water and carbon cycles to origins of the Earth’s oceans and diamonds, to name a few. Recent advances in high-pressure laser and synchrotron X-ray spectroscopic techniques now permit direct examinations of the microscopic properties of earth materials at pressure-temperature conditions of the deep Earth. Similar to solid-state physics methods, these mineral physics studies consider how the large-scale processes of earth materials result from their atomic-scale properties occurring deep inside the Earth. In this talk, I will use my research results from diamond anvil cell experiments as examples to highlight these solid state geophysics efforts. Specifically, I will show how recently observed electronic spin transitions of iron, the most abundant transition metal in the Earth, affect seismic and transport properties of the lower mantle, how phonon dispersions of iron alloys have been applied to decipher the composition and formation of the remotest core, and how physicists and geophysicists are working closely to investigate long-range electron-electron interactions that may help address both particle physics and geophysics questions. Future challenges and research opportunities in materials properties under extreme environments will also be presented so as to stimulate students and researchers to explore this new frontier collaboratively.

Oct. 10 - Prof. Larry Hunter, Amherst College

"Using the Earth as a Polarized Electron Source to Search for Long-Range Spin-Spin Interactions"

Many extensions of the standard model of particle physics predict the existence of long-range spin-spin interactions. We have developed an approach which uses the Earth as a polarized spin source to investigate these interactions.We combine recent deep-Earth geophysics and geochemistry results with precise tabulations of the geomagnetic field to create a comprehensive map of electron polarization within the Earth.We examine possible long-range interactions between these spin-polarized geoelectrons and the spin-polarized electrons and nucleons in three laboratory experiments.By combining our model and the results from these experiments we establish new stringent bounds on torsion gravity and possible long-range spin-spin forces associated with the virtual exchange of either spin-one axial bosons or unparticles.The resulting bound on the spin-spin force between an electron and a neutron is one million times smaller than their gravitational attraction.

Oct. 17 - Dr. George Basbas, Editor, Physical Review Letters

"Why Shakespeare Must Have Been a Physics Journal Editor Before Becoming a Playwright"

After briefly describing the larger context of the physics journals of the American Physical Society, the focus will fall upon Physical Review Letters in particular. PRL is a high profile journal which publishes leading-edge research results in physics the world over. What goes on when authors attempt to publish their work in such a journal, and how an editor deals with it, will be illustrated. Evidence to support the contention of the talk title will be adduced.

Oct. 24 - Prof. Robert Bluhm, Colby College

"Testing Relativity"

The theory of relativity has held for over a century. However, general relativity is incompatible with quantum physics at very high energy scales known as the Planck scale, and it is expected that it will eventually merge into a quantum theory of gravity, such as string theory. Investigations in quantum gravity suggest that small violations of relativity might occur in this context. This talk will explore how such violations might be searched for using an extension of the standard model of particle physics that incorporates small relativity violations. An overview of this framework and some of the experimental tests will be described.

Nov. 7 - Dr. Michael Stage

"High-Energy Physics of Supernova Remnants"

Galactic supernova remnants (SNRs) radiate strongly in X-rays, through both thermal and nonthermal processes. The advanced X-ray imaging and spectroscopic telescopes of the last decade, the Chandra X-ray Observatory, XMM-Newton, and the Suzaku X-ray Telescope, have allowed the study of the remnants in much greater detail than ever before in the 1-10 keV band. After a brief introduction to supernovae and the telescopes used to study their remnants, I will describe some of my own research, with an emphasis on the basic physics behind the phenomena. Using spatially-resolved spectroscopic analysis, my colleagues and I have been able to separate the synchrotron radiation of accelerated electrons in a remnant's forward shock from the line and thermal continuum radiation produced by the stellar ejecta. Observations of the synchrotron radiation emitted by electrons has provided strong evidence that, as has been long believed, Galactic cosmic rays are accelerated via diffusive shock (Fermi) acceleration in SNRs, and in some locations it occurs as efficiently as possible (the Bohm limit). I will also describe how we use the observations in the 1-10 keV band to better understand the higher-energy (10 - 80 keV) spectra of remnants and to deduce properties of the populations of ions and electrons in remnants.

Nov. 12 - Dr. Christian Bracher, Bard College

"How (Small) Things Move:Physics at the Intersection of the Classical and Quantum Realms"

Physics uses two conceptually very different theories to describe the motion of objects:For “macroscopic” things like cannonballs, the model of choice is classical mechanics, which goes back to Galilei and Newton, and analyzes the position of the object as a function of time, i.e., its trajectory.In the microscopic universe of electrons, however, quantum mechanics prevails - the counter-intuitive yet phenomenally successful theory developed by Schrödinger, Heisenberg, and many others that assigns wave-like behavior to all objects.At first glance, the two descriptions seem wholly incompatible. What gives?

In my talk, I'll first introduce the ideas behind the semiclassical approximation, an ingenious combination of the two approaches, and illustrate it using ballistic motion as an instructive example.Then, I will apply this scheme to the dynamics of electrons in electric and magnetic fields, and present some surprising results that my students and I recently obtained using the semiclassical technique.

Nov. 14 - Dr. Michael Ray, Post-Doctoral Researcher, Amherst College

"What can you do with a Bose-Einstein Condensate?"

Nearly twenty years ago the first Bose-Einstein condensate (BEC) was observed in dilute atomic gases. Since then the field has expanded into areas that could never have been imagined back then. BEC's are now used to explore everything from atomic physics to condensed matter systems, and can be used to construct some of the most sensitive interferometers. Using BEC's for these purposes is advantageous due to the simplicity of the system, and the fact that they can be made on a table top without the use of cryogens.

In this talk I will first present a brief overview of the phenomenon of Bose-Einstein condensation, and discuss different areas where they can help us to understand more complex phenomena with the focus on what happens when a BEC rotates. Then, I will review the idea of Dirac monopoles, and show how they can be created in a BEC. I present evidence that such a particle has been observed here at Amherst.

Nov. 21 - Dr. Keisuke Hasegawa, National Institutes of Health,

"Understanding molecular self-assembly in live cells using FRET microscopy"

Self-assembly is a process in which a disordered system of components forms an organized structure without external intervention. Self-assembly is ubiquitous in nature especially in biological systems. A striking example in biology is the self-assembly of the mitotic spindle, a macromolecular machine that segregates chromosomes in a dividing cell. To regulate the spindle self-assembly, a mitotic cell generates a concentration gradient of RanGTP and creates a spatial bias for spindle formation around the chromosomes. Until recently, the RanGTP-regulated spindle assembly has been studied in cancer cells and few other model systems, but not in normal healthy cells. In this talk, I will describe Föster resonance energy transfer (FRET) biosensors we have developed to visualize the RanGTP gradient in live cells in order to investigate its universality. I will also discuss biological and potential biomedical implications of our findings.

Shah Saad Alam, "Studying the hyperfine spectra and branching ratios of the B(0)-X(0) transition of TlF"

Dec. 14 - Reading Period

Dec. 21 - Winter Recess

Spring 2014

Jan. 10 - Prof. Douglas Leonard - San Diego State

"A Supernova Riddle"

We know that it happens, but we don't know how Nature does it. Roughly once per second in the observable universe, a massive star's inner core implodes under its own weight and then explodes as a supernova, announcing its demise with an optical display that for weeks rivals the combined brilliance of all of the other stars in its parent galaxy. These explosions synthesize and expel heavy elements, heat the interstellar medium, trigger vigorous bursts of star formation, create neutron stars and sometimes black holes, and produce energetic cosmic rays. And yet we must acknowledge basic ignorance: We do not know how -- or even precisely which -- stars explode. Using recent observations that demand the resolving power of the Hubble Space Telescope and the light-gathering capability of the world's largest ground-based telescopes, I will argue that despite the mystery that enshrouds these cosmic blasts, fundamental advances on both fronts are being made.

Jan. 14 - Dr. Jessica Werk - UCSC

"The Invisible, Large Reservoirs of Gas Around Galaxies"

In a galaxy like the Milky Way, its tens of billions of stars are its most visible, widely-recognized constituents. The stars dominate the energetic output of a galaxy and ultimately define its standing in the pantheon of modern-day galaxies. Another crucial, though less-visible, component of a galaxy is its interstellar medium (ISM). The ISM provides the gas to fuel the formation of new stars and collects the material by-products from the stars that die explosive deaths. Engulfing both of these components, and dwarfing them in spatial extent and mass, is an enormous halo filled with non-baryonic dark matter that cannot be seen directly. Only in the last decade have we been able to detect and recognize yet another invisible, massive component of galaxies: the circumgalactic medium (CGM). The CGM plays a unique and vital role in galaxy formation and evolution, and we are only just beginning to understand its importance. In this talk, I will describe this elusive medium, its discovery, and the observational techniques I am using to characterize it.

Jan. 16 - Dr. Daryl Haggard, Northwestern

"News from the Galactic Center: Constraints on a Collision between the Milky Way's Supermassive Black Hole and the "G2" Gas Cloud"

The recent discovery of a dense, cold cloud (dubbed "G2") approaching the super massive black hole at our Galactic Center offers an unprecedented opportunity to test models of black hole accretion and its associated feedback. G2's orbit is eccentric and the cloud already shows signs of tidal disruption by the black hole. High-energy emission from the Sgr A*/G2 encounter will likely rise toward pericenter (early 2014) and continue over the next several years as the material circularizes. This encounter is also likely to enhance Sgr A*'s flare activity across the electromagnetic spectrum. I will present preliminary results from joint Chandra/VLA monitoring campaigns in 2013 (>400 ks from Chandra and ~30 hours from VLA). These programs aim to study the radiation properties of Sgr A* as G2 breaks up and feeds the accretion flow, to constrain the rates and emission mechanisms of faint X-ray flares, and to detect G2 itself as it is shocked and heated. I will also outline plans for continued monitoring with Chandra, XMM, HST, Spitzer, and VLA in 2014.

Cosmological N-body simulations predict that there should be many hundreds of low-mass dark-matter haloes surrounding the Milky Way galaxy, while the number of known satellite galaxies of the Milky Way is only a few dozen. This large discrepancy between theory and observation is often referred to as the "missing dwarfs problem." The resolution of this problem that is commonly invoked is that the low-mass haloes have either lost all of their normal matter (e.g., baryons and electrons) or that the normal matter is too hot to form into stars. In this talk I describe a new survey that is detecting small amounts of cold hydrogen gas in nearby dwarf systems, most of which do not have an obvious stellar component. Our recent deep optical imaging has discovered at least one example of an extremely low-mass galaxy located in the outskirts of the Local Group that may represent an example of the population of "missing" dwarfs.

Jan. 28 - Dr. Alison Crocker, University of Toledo

"The price of stars: exploring star formation efficiencies in different galaxies"

The galaxies in the present-day universe are all composed of millions or billions of stars, making star formation one of the crucial process of galaxy evolution. An important question is what determines how many stars are formed out of a given mass of interstellar gas, per unit time. This quantity is called the star formation efficiency and has now been measured in many different types of galaxies from local spirals to distant starbursts. While the surface density of the gas is suggested to be the main variable determining star formation efficiency, other galaxy properties also seem to have an effect. Towards the end of the talk, I will focus on the star-formation efficiencies of early-type galaxies. This is a new area for star formation research, because until recently, early type galaxies were considered purely quiescent systems, with no ongoing star formation. Helpfully, star-forming early-type galaxies have several unique properties that likely influence how star-formation proceeds and thus can further our understanding of this important process.

Feb. 11 - Dr. Aleks Diamond-Stanic, University of Wisconsin-Madison

"The Universe Needs Feedback: Problems with the Cosmic Fuel Supply"

We live in a universe where only 5% of the mass-energy is in the form of normal matter; the remainder is attributed to non-baryonic dark matter and dark energy. Furthermore, only 5% of the normal matter has managed to form a star or planet by the present day, which is an order of magnitude less than predicted by numerical simulations of galaxy formation. This discrepancy challenges our understanding of what galaxies do with their supply of gas, the fuel for star formation. I will discuss how my research with ground- and space-based telescopes addresses this discrepancy by studying how gas gets into galaxies, how it participates in star formation and black hole growth, and how it is returned to its galactic surroundings via feedback. In the process, we are learning about how radiation, momentum, and thermal energy from stars and black holes regulate the fuel supply for star formation, and why we are only 5% of the 5%.

Feb. 13 - Reserved, TBA

Feb. 20 - CANCELLED Prof. Christopher Landee, Clark University

Feb. 27 - Prof. Enrique del Barco, University of Central Florida

"Quantum Tunneling of the Magnetization in Molecular Magnets: A Microscopic View"

Intensive research efforts during the last few years have led to a significant enhancement of the knowledge of magnetic bistability and quantum tunneling of the magnetization (MQT) in single-molecule magnets (SMMs). A good part of this new understanding has been allowed by studies of families of relatively simple, low-nuclearity transition metal clusters, where the low number of ions responsible for the magnetic behavior of the molecule enables a full numerical treatment of the multi-spin Hamiltonian, where degrees of freedom associated to the individual ions can be adequately tested. These include exchange and dipolar interactions, single-ion zero-field splitting tensors (i.e. single-ion anisotropy order and orientation), and molecular symmetries, among others. In addition, comparisons between the giant spin and the multi-spin phenomenologies can be easily undertaken. Note that the giant spin approach assumes the molecule to behave as a rigid spin unit, leaving out the intra-molecular degrees of freedom, and ultimately failing to capture many of the key physics. In this talk I will summarize the most relevant results achieved by following this particular research approach, including the first observation of QTM selection rules that reflect the intrinsic symmetry of a SMM [1,2,3], and (time permitting) the observation of asymmetric quantum Berry phase interference patterns in a Mn-based SMM [4], placing special emphasis in explaining the way internal molecular degrees of freedom intervene to shape these fundamental QTM properties.

More than 80 years after its proposed existence, the neutrino remains largely mysterious and elusive. Despite this fact, we are closing in on answers to some of the big questions surrounding the "little neutral one". After an introduction to the neutrino and neutrino mass, I will discuss one of the most important open questions in particle physics and cosmology: Is there a difference between matter neutrinos and antimatter neutrinos?

March 11 - Dr. Nick Cowan, Northwestern University

"Planetary Science from the Top-Down: The Exoplanet Opportunity"

What started as a trickle in the mid 1990's is now a torrent, with over one thousand extrasolar planets currently known, and thousands of candidates awaiting confirmation. The study of exoplanets has already revolutionized our view of planet formation, and will soon do the same to our understanding of planetary atmospheres and interiors. The diversity of exoplanets gives us the leverage to crack hard problems in planetary science: cloud formation, atmospheric circulation, plate tectonics, etc. However, the characterization of exoplanets presents a challenge familiar to astronomers: our targets are so distant that we only see them as unresolved dots. I will describe how we can extract spatially-resolved snapshots of planets from such observations. These data are sufficient to constrain low-order climate models and therefore give us insight into the effects of clouds, heat transport, and geochemical cycling. Coarse measurements for a large number of planets is the perfect complement to the detailed measurements possible in the Solar System. That is the exoplanet opportunity.

March 13 - Dr. Sarang Gopalakrishnan '06, Harvard University

"Thermalization and Its Discontents"

How -- and in what sense -- does a large, isolated quantum system approach thermal equilibrium? I will review recent experimental and theoretical developments that have revived this old question, focusing on systems in which the approach to equilibrium is either anomalously slow or altogether absent. I will then introduce a set of arguments that suggest that the absence of intrinsic equilibration is a general feature of strongly disordered quantum systems, which undergo a phase transition called the "many-body localization" transition. Finally, I will discuss the unusual properties of many-body localized states, such as the persistence of quantum coherence at high temperatures.

All molecular forms of life are handed due to the fact that they possess a chiral asymmetry. But the question of why some molecules, such as DNA, exist in only one chirality for all forms of life is, as of yet, unanswered. One possible explanation, known as the Vester-Ulbricht hypothesis, suggests that chiral electrons from beta-radiation preferentially destroyed one of the two enantiomers. I will present experiments that provide evidence for this hypothesis. The first experiment considers the transmission of polarized electrons through chiral molecules, and the other studies the break-up of chiral molecules by polarized electrons in a dissociative electron attachment process. The results of both demonstrate the existence of vital asymmetries in the interactions.

March 27 - Prof. Christian Santangelo, University of Massachusetts

"2D or Not 2D: Folding and Buckling Flat Sheets to Make 3D Structures"

Origami, the ancient art of folding paper, has long been used as a means to create three-dimensional sculptures from a flat sheet of paper. From the perspective of manufacturing, it is often easier (and cheaper!) to fold a flat sheet of material into a 3D object. In addition to its shape, the folded sheet often exhibits mechanical properties not present before folding. I will discuss the mechanics and shape of various origami structures, and finally discuss our theoretical and experimental work creating self-folding origami materials. Though still in its infancy, these new materials hold promise in the fabrication of new responsive materials not easy to achieve in conventional materials.

April 3 - Prof. Brian Tiburzi '99, City College of NY|

"A Smaller Size for the Proton?"

A century ago, the spectrum of the hydrogen atom posed a serious puzzle whose solution helped launch the quantum-mechanical revolution. Today, there is a new puzzle concerning the hydrogen atom. This puzzle concerns the size of the proton, which has recently been determined to the highest precision ever from spectroscopy of muonic hydrogen, and is significantly smaller than other proton-size measurements. Could this smaller size for the proton amount to a momentous discovery? Can large-scale numerical calculations determine the proton size directly from the theory of strong interactions? I will address these questions in light of world-wide efforts to resolve the proton-size puzzle.

North American unconventional oil and gas production has increased dramatically over the past several years. This production relies on hydraulic fracturing — injecting large quantities of fracturing fluid composed of water, sand, and a suite of chemical additives into shale or tight sands formation at high pressure creating fractures that stimulate fluid flow. There are many significant environmental impacts from this activity but the public in many states has been particularly concerned with the possibility that the chemical additives can migrate into water supplies. The public expects state and federal regulators to require disclosure of these chemicals but an effective and practical scheme has proven illusive. What are the issues and what should be done?